Ako večeras operem novu plahtu, sutra ću je staviti na krevet prije nego što gosti dođu.

Because oprati / operem is the perfective verb, while prati / perem is imperfective.

In this sentence, the speaker means if I wash the sheet this evening and finish doing it, then tomorrow something else will happen. Croatian often uses the perfective present in clauses like this to talk about a completed future action:

• Ako večeras operem... = If I wash it this evening / if I get it washed this evening

If you said Ako večeras perem..., that would sound more like if I am washing / if I wash as an ongoing activity, which is not the natural choice here.

Croatian very often uses the present-tense form after words like ako when talking about the future.

So:

• Ako večeras operem novu plahtu... literally looks like present tense
• but it means If I wash the new sheet this evening...

This is normal in Croatian. English often uses present tense after if too:

• If I wash it tonight, I’ll put it on the bed tomorrow.

So the grammar idea is similar, even though Croatian also uses aspect very strongly here.

Because Croatian clitics have a fixed position pattern, and ću and je are both clitics.

In neutral standard word order, the future auxiliary usually comes first in this clitic group, and the object pronoun follows it:

• sutra ću je staviti

Not:

• sutra je ću staviti

So the order here is natural and standard.

Je here means it.

It refers back to novu plahtu. Since plahta is a feminine singular noun, the accusative singular pronoun is:

• je = it / her

So:

• sutra ću je staviti na krevet = tomorrow I will put it on the bed

Even though je can also mean is in other sentences, here it is clearly the object pronoun.

Because this is the direct object of the verb operem, so it must be in the accusative case.

The dictionary form is:

• nova plahta

But in the accusative singular feminine, both words change:

• nova → novu
• plahta → plahtu

So:

• operem novu plahtu = wash a/the new sheet

Because Croatian distinguishes between:

• motion toward/onto something → usually na + accusative
• location on something → usually na + locative

Here the meaning is put it onto the bed, so there is movement:

• staviti na krevet = to put onto the bed

Compare:

• Plahta je na krevetu. = The sheet is on the bed.
  Here there is no movement, so na krevetu is used.

For the same reason as operem: aspect.

• staviti = perfective, to put/place something once, as a completed action
• stavljati = imperfective, to be putting / to put repeatedly or habitually

In this sentence, the speaker means one completed future action:

• sutra ću je staviti na krevet = tomorrow I’ll put it on the bed

So staviti is the natural choice.

Because Croatian commonly uses the present form in subordinate clauses to refer to future events, especially with a perfective verb.

• doći / dođu is perfective
• prije nego što gosti dođu = before the guests come / arrive

The idea is before they arrive, meaning before that future action is completed.

Because dođu comes from the perfective verb doći, while dolaze comes from the imperfective verb dolaziti.

Here the meaning is before the guests arrive as a single completed event, so Croatian prefers the perfective:

• prije nego što gosti dođu

If you used dolaze, it would suggest an ongoing or repeated action and would not fit as well in this context.

Prije nego što means before in the sense of before something happens.

In many contexts, speakers may also say prije nego without što, and you may hear both. But prije nego što is very common and completely standard.

So:

• prije nego što gosti dođu
• prije nego gosti dođu

Both can be heard, but the version with što is very common in careful standard usage.

Because in Croatian, many common time expressions work without a preposition.

For example:

• večeras = this evening / tonight
• sutra = tomorrow

So Croatian simply says:

• Ako večeras operem...
• sutra ću...

This is very normal and does not need a word like on or in.

Yes, Croatian word order is flexible, but not completely free. This sentence could be rearranged for emphasis, while the clitics still have to follow their usual rules.

For example, the neutral version is:

• Ako večeras operem novu plahtu, sutra ću je staviti na krevet prije nego što gosti dođu.

A different emphasis might give:

• Sutra ću je staviti na krevet ako večeras operem novu plahtu, prije nego što gosti dođu.

However, the original version sounds very natural and clear. Learners should usually stick close to that pattern until they get used to Croatian clitic placement and emphasis.

Usually, no. After ako for a real condition, Croatian normally uses a present-tense form, often with the appropriate aspect.

So:

• Ako večeras operem... = correct

Not normally:

• Ako ću večeras oprati...

The main clause can then use the future:

• sutra ću je staviti...

So the pattern is:

• Ako + present
• main clause + future

Gosti is the nominative plural form of gost.

It is the subject of dođu, so nominative is required:

• gosti dođu = the guests come / arrive

Singular would be:

• gost dođe = the guest comes / arrives

Plural:

• gosti dođu = the guests come / arrive